Recently recognized environmental concerns have suggested a need to measure the chemical constituents, carried components, qualities and characteristics of the air (hereinafter components) moving down from the atmosphere toward the earth's surface and up from the earth's surface toward the atmosphere. Vertical "flux" is a term used to describe the quantity of a selected component carried by the air or a gas passing through a given horizontal level or area in a given time. Among other things, flux information is used to calculate and predict environmental impacts on the earth's surface and to the atmosphere, such as acid rain, ozone, methane and carbon monoxide, as examples. Meteorologists employ flux measurements in deriving and predicting certain weather and meteorological information, such as influences from temperature, humidity and airborne particulate matter, as examples. A variety of other known applications for flux measurement now exist.
Simply stated, flux measurement involves sensing the vertical velocity of the gas or air movement, measuring the concentration of a desired component carried by the air in each direction of sensed air movement, multiplying the vertical air velocity by the component concentration measured to obtain the amount or quantity of the component transferred in each direction, and algebraically summing the measured amounts transferred upward and downward over a period of time to obtain the net amount of the component transferred in one direction or the other, i.e. the flux of that component. Although simple enough to understand in concept, a number of practical difficulties have made accurate flux measurement for all types of components impossible or extremely difficult to achieve.
One known flux measurement technique is the instantaneous or eddy correlation technique. The instantaneous technique follows the procedure outlined above, but its application is limited to those components for which there are sensors capable of instantaneously, or very rapidly, measuring the concentration of the desired component. This is a serious limitation because the sensors which are capable of instantaneous measurement are responsive to only a few of the components for which flux measurements are desired. For example, flux measurements of temperature and humidity can be instantaneously achieved, because temperature and humidity sensors capable of measuring or responding on a nearly instantaneous basis are currently available. Instantaneous measurements are required because the measured concentration of the component must be correlated to the vertical velocity of the air movement, which may fluctuate rapidly and repeatedly over the course of time while the flux measurement occurs. Presently, it is believed that no more than about five to ten different types of sensors exist which are capable of responding instantaneously for flux measurement purposes. Accordingly, the instantaneous technique is severely limited in its applicability, because scientists desire to measure the flux of many more components than just those few components for which instantaneously-responsive sensors are available.
To avoid the problem or limitation created by the unavailability of instantaneous sensors, a component sampling technique known as eddy-accumulation has been developed for use in flux measurement. The eddy-accumulation sampling technique uses two reservoirs or canisters to collect samples of air containing the component, and the air samples are later analyzed by laboratory techniques which do not require instantaneous sampling. One canister is used to accumulate samples from the upward moving air, and the other canister accumulates samples from the downward moving air. In order for the eddy-accumulation sampling technique to be accurate, the rate of sample accumulation in each canister must be proportional to the magnitude of the vertical velocity. Proportional accumulation in each canister is achieved by a valve which attempts to quickly and accurately vary the size of the passageway into the canister in proportion to the magnitude of the vertical velocity of the air.
The drawback of eddy-accumulation sampling lies in the practical difficulty of accurately controlling the size of the passageway to the canisters. This problem has proved to be so difficult to overcome that flux measurements from eddy-accumulation sampling have not been accepted as fully reliable or accurate. Thus, while the eddy-accumulation sampling is theoretically available to determine fluxes of a wide variety of components, its accuracy is problematical. Consequently flux measurements based on eddy-accumulation sampling have met with only very limited acceptance.
It is with regard to the desire to achieve accurate component sampling and a flux measurement technique that obtains the advantages and avoids the disadvantages of the instantaneous and eddy-accumulation sampling techniques, that the present invention has resulted.